This invention relates generally to delay devices used in electronic circuits, and more specifically, to a radio frequency (RF) delay device and system for radar altimeter calibration.
Many aircraft require better accuracy from a radar altimeter than presently exists. Generally, the accuracy becomes more important at low altitudes where aircraft perform controlled flight into and just above terrain. For example, accuracy becomes more important during landing, low altitude equipment drops, precision hovering, detection avoidance, and nap of the earth flying. Some of these applications include unmanned vehicles where landing is controlled remotely and there is little room for error. The low altitude region of a radar altimeter, where the accuracy becomes more important, is usually defined as from 0 to 50 feet. Laser systems have been proposed but problems, for example, with weather, errors relative to aircraft attitude with a collimated beam, and inability to see through dust, rain, fog and other environments have negated their use for critical radar altimeter applications.
The total accuracy of a radar altimeter system is a function of sensor accuracy and ground return signal accuracy. Sensor accuracy is diminished by variations due to environmental changes, including but not limited to changes in temperature and humidity, and affected by variations in signal amplitude, risetime, bandwidths, pulse or gate widths, and clock frequencies.
In contrast to sensor accuracy where the error is caused by variations within the radar altimeter system, ground return signal accuracy is a function of the radar signal from when it leaves a transmit antenna to when it is received at a receive antenna. Ground return signal errors are caused by vehicle attitude, the external environment including but not limited to rain, fog, and dust, and terrain characteristics and associated reflection coefficient characteristics including shaping functions. The above described errors are difficult to detect and correct in a radar altimeter. As a result, wide accuracy tolerances are utilized to account for the various error sources.
Radar altimeters are currently tested for accuracy by either acoustic or optical delay lines. These delay lines are external of the radar altimeter and used mostly in production testing rather than while a radar altimeter is in use. These delay lines are also very large, often larger than a radar altimeter itself, and expensive.